Why did President John Tyler?

Answer 1

His term of office expired and he was nominated for re-election.

Answer 2

John Tyler's presidency was tumultuous and viewed by most historians as a failed presidency. However, the only legacy that lasts until today is his establishing the precedent that presidents who enter office upon the deaths of their predecessors are both legitimate and unbound by their predecessor's policies. When Harrison died, it was unclear what the Constitution meant in its original wording on succession. In fact, Harrison's cabinet thought they would run the country themselves, as a committee - in their view, Tyler was just a figure-head, an "acting President" who simply filled the office until the next election. Tyler, however, made it quite clear that he was, in fact, the president. Although his vetoes of Whig legislation was seen then and now as unwise as matters of policy, the vetoes had the effect of establishing him as a bona fide president independent of Harrison. Later presidents who assumed office on the deaths of predecessors would often publicly say they would continue their predecessor's policies but would soon make it clear through their actions that they were unbound by such.

A final note: Tyler believed that the office devolved upon him at the moment of Harrison's death and taking the Oath of Office was therefore unnecessary. He was convinced to do so anyway a few days later but he wrote a public letter stating his beliefs. Since then, the established principle is that successors do in fact become the president at the moment the predecessor leaves office, whether through death or the end of a term. When in 2009 the Oath of Office was flubbed during the inauguration, Barack Obama was still considered to be the president at noon when George W. Bush's term expired (the oath took place a few minutes after noon). A few days later, the oath was readministered "out of an abundance of caution," in the words of one administration lawyer - a fitting paraphrase for how Tyler was himself convinced to take the Oath.

Answer 3

Vice-president John Tyler became president due to the death of President William Henry Harrison, who was only president for 4 weeks.

Answer 4

William Henry Harrison was president at first. But he then died of a stroke and so John Tyler became president since he was vice president beforehand.

Answer 5

John Tyler was elected as Vice President, but became President when William Henry Harrison died of pneumonia.